Alcohol is the most widely used licit drug and marijuana the most widely used illicit drug among emerging adults and each is associated with increased sexual risk taking behavior. The overall goal of this project is to reduce Sexually Transmitted Infections (STI) among emerging adults (18-25 years old) and to advance knowledge of the relationship between binge alcohol use, marijuana use, and sexual risk-taking. More specifically, the aim of this study is to demonstrate a reduction in STIs and sexual risk behavior through the reduction of alcohol and marijuana use. We propose a randomized clinical trial in which 500 sexually active, emerging men and women will be assigned to either (a) a brief, motivationally-focused alcohol and marijuana intervention (MI), or (b) a health education (HE) attention-matched control condition. The Primary Aims of the present application are: 1. To test the hypothesis that among emerging adults who are engaging in alcohol and marijuana use, a brief motivational intervention (MI), relative to health education (HE), will result in lower STI incidence after 9 and 15 months. 2. To test the hypothesis that among emerging adults who are engaging in marijuana and alcohol use, MI will result in less alcohol use relative to HE, as indexed by lower percentage of alcohol use days after 9 and 15 months. 3. To test the hypothesis that among emerging adults who are engaging in marijuana and alcohol use, MI will result in less marijuana use relative to HE, as indexed by lower percentage of marijuana use days after 9 and 15 months. 4. To test the hypothesis that alcohol and/or marijuana mediate the effect of MI on STI outcomes. The outcomes for this study will be STIs measured by biologic testing, as well as alcohol and marijuana use days assessed at 9 and 15-months. This project continues a program of research begun with our previous brief motivational intervention trials. While prior research, including our own, demonstrates that MI can reduce alcohol or marijuana use, previous work has not focused on the potentiating effects of these two substances in combination. Here, we use emerging adulthood development theory to structure our intervention aimed at reducing alcohol use and marijuana use and designed to maintain these effects in order to decrease STI acquisition among the large population of community-based emerging adults. In the absence of intervention, these emerging adults are at heightened risk of contracting sexually transmitted infections. If the current intervention is found to be efficacious in this broad sample, it can be readily integrated into a variety of treatment settings where emerging adults seek care, such as college health services, gynecological and primary care practices, and behavioral health settings; furthermore, a future version of this intervention could be internet-based.